In the field of analysis of gene expression the behavior of those genes which determine the character of a living thing can be studied by incorporating selected genes into living cells and by examining the transformation of the cells having added genetic material. For example, DNA can be extracted from cancerous cells, and divided into minute fragments of different sizes. These fragments are classified in terms of size, and fragments of different size are incorporated into living cells. As a result of such incorporation, some of the cells are found to be cancerous, presumably as a result of incorporation of the foreign DNA fragments. By this technique, cancer-associated fragments can then be identified in terms of size.
A prior art implantation method comprises the steps of: putting living cells in a solution containing DNA fragments; making a small hole in each living cell with the aid of a fine needle under an optical microscope; allowing DNA fragments to enter the cells through the hole; and confining the fragments in the cell when the hole in the cell heals (See Japanese Patent Application No. 56-171347, disclosed as Public Disclosure Number 58-76091 on May 9, 1983). Another prior art implantation method comprises the steps of: precipitating DNA using calcium phosphate in a culture medium; and making use of the phagocytosis of living cells to incorporate the precipitated DNA into the living cells.
The prior art implantation methods, however, are not entirely satisfactory. The former implantation method requires skilled manipulation of the needle. Otherwise, no holes can be made without injuring the living cells. Also the work is tedious and laborious although the implantation succeeds at a relatively high rate. Furthermore, it is impossible to make holes in certain cells, regardless of the type of needle used.
The latter implantation method is capable of handling a great number of cells at one time. The success rate for incorporation of DNA fragments into host cells by this method, however, is very low, say one in ten thousand (1/10,000) at best. Thus, the rate at which the character of the implanted cells is transformed is very small, and accordingly a very large number of cells need to be implanted with DNA fractions. This demand cannot be met. Also, disadvantageously the method requires addition of calcium phosphate of so high a concentration that the additive tends to injure living cells.